In a scanning transmission electron microscope (STEM), a focused beam of high-energy electrons is scanned across a thin sample. Electrons in the beam interact with the sample as they pass through it and are collected below the sample. Some electrons pass through the sample relatively unhindered; others are deflected, absorbed, or lose energy. Different imaging and analysis techniques use different characteristics of the transmitted electrons to form an image or to determine properties of the sample. The term “STEM imaging” is used herein to refer to obtaining information about the sample from the number of the electrons impacting a detector as a focused electron beam is scanned along the sample surface. A “STEM detector” used in STEM imaging may be a scintillator-photomultiplier detector, known as an Everhart-Thornley detector, a PIN solid state detector, or any other suitable detector. A STEM detector is typically sufficiently fast to allow data collection as the primary electron beam scans a region of the sample surface. A typical scan, for example rastering the beam over a thousand rows with a thousand scan points in each row, may take about one second and can generate over a million pixels of information, and so the detector preferably can provide readings at a rate of at least one million readings per second. A typical high accuracy STEM imaging circuit can obtain a reading in microseconds, preferably less than 100 μs, less than 50 μs, and more preferably less than 10 μs.
Electrons that pass through the sample without a large change in direction can be detected along the beam axis in a process referred to as “bright-field STEM imaging.” Electrons that are deflected through larger angles by the sample can be collected away from the optical axis in a process referred to as “dark-field STEM imaging.” Electrons for bright-field and dark-field STEM imaging can be detected by detectors that produce a signal proportional to the number of impinging electrons. A STEM detector for bright-field imaging is typically circular and centered on the system optical axis. A STEM detector used for dark-field imaging is typically annular and concentric with the axis. Information from bright-field imaging or dark-field imaging collected at each point in the scan of the primary electron beam can be used separately or together to provide an image or to map a characteristic of the sample.
In another mode of imaging, so-named TEM imaging, the sample is irradiated with a parallel beam of electrons. Again, part of the electrons pass through the sample relatively unhindered; others are deflected, absorbed, or lose energy. An image of the sample is formed on a detector, the detector in the form of e.g. a fluorescent screen, a CMOS camera, a CCD camera, or any other suitable detector.
It is noted that many instruments are capable to form TEM images and STEM images.
Additional information about the sample can be provided by measuring the energy absorbed as electrons pass though the sample. This technique is called “electron energy-loss spectroscopy” or EELS. An overview of EELS is provided by R. F. Egerton in “Electron energy-loss spectroscopy in the TEM,” Reports on Progress in Physics 72 (December 2008). Different materials in the sample cause electrons to lose different amounts of energy as they pass through. The electrons pass through a spectrometer to determine the energy loss by subtracting their exiting energy from the electron energy in the original electron beam. EELS can determine not only which elements are present, but also their chemical states.
An EELS spectrometer typically includes one or more prisms that separate electrons by their energies in an energy-dispersive plane by deflecting the electrons by an amount that depends on the electron energy. An energy dispersive plane is a plane in which electrons having different energies are dispersed in a direction normal to the direction of the beam travel. The term “prism” as used herein means any device that disperses the electron beam depending on the energies of electrons in the beam. A prism can provide, for example, a magnetic or electric field perpendicular to the beam. For example, a portion of a spherical capacitor, a magnetic deflector, or a Wien filter can be used as a prism. The angular dispersion of the electrons depends on the strength of the magnetic or electric field in the prism and the energy of the electrons. A prism may comprise multiple elements. Beside a prism, an EELS spectrometer may also include an adjustable energy-selecting slit, typically positioned in or near the energy-dispersive plane, and imaging optics that may include a system of prisms and/or lenses and/or multipoles or combinations thereof, to form an electron image on a detector that records the image. The detector can be, for example, a charged coupled device or active pixel sensor and may include a row or a two-dimensional array of pixels. Projection optics positioned after the sample and before the spectrometer project electrons into the entrance aperture of the spectrometer.
Typically, EELS spectrometers can be operated in two modes. In the first mode, called the “spectroscopy mode,” the imaging optics form an image of the energy-dispersive plane on the detector. In this way, the image on the detector constitutes a spectrum of the energy lost in the specimen. The system of prisms and/or lenses and/or multipoles between the energy dispersive plane and the detector can be used to vary the magnification of the energy-dispersive plane on the detector. Low magnifications can be used to image an overview of the spectrum, and high magnifications can be used to image details in the spectrum. In this mode, the energy-selecting slit is usually not needed, and it is set sufficiently wide that its shadow is not visible on the detector.
In the second mode of operation of EELS spectrometers, called the “energy-selected” or “energy-filtered” imaging mode, the imaging optics form an image of the entrance plane of the spectrometer on the detector. The projection optics between the sample and the spectrometer can be set so that the entrance plane of the spectrometer contains a TEM image of the sample, and then the detector plane contains again a TEM image of the sample. Alternatively, the projection optics can be set so that the entrance plane of the spectrometer contains an image of the back-focal plane of the first lens after the sample, which image is commonly referred to as a diffraction pattern of the sample. The energy-selecting slit can be used to form a TEM image or diffraction pattern on the detector which is formed only by electrons which have lost a specific amount of energy passing through the sample.
Some EELS spectrometers cannot operate in this second mode (for example, because the detector is not capable of recording two-dimensional images, or because the system of lenses or multipoles is not flexible enough to form an image of the entrance plane), and therefore these EELS spectrometers do not require an energy-selecting slit.
There are several mechanisms by which electrons lose energy as they pass through a sample. The different mechanisms cause electrons to lose different amounts of energy and account for the shape of a typical energy loss graph or spectrum. FIGS. 1A and 1B are spectra that show in arbitrary units numbers of electrons detected at various energy loss values. The energy loss spectrum varies with the material present in the sample and so information about the sample can be inferred from the spectrum.
FIG. 1A shows the so-called “low-loss” region 100 of the energy loss spectrum, which is defined somewhat arbitrarily as regions of less than 100 eV. Electron losses in the low-loss region result primarily from inelastic interactions, such as phonon interactions, plasmon interactions, collisions with outer shell electrons, non-ionizing collisions with inner shell electrons, and radiation losses. FIG. 1B shows a typical “core loss” region 108 of the spectrum. Electron losses in the core-loss region result from ionization of inner shell or “core” electrons and losses are typically greater than 100 eV. The spectra of FIG. 1A and FIG. 1B are not drawn to the same scale; the vertical scale of FIG. 1B is much enlarged compared to FIG. 1A.
FIG. 1A shows a large peak 102, called the “zero-loss peak,” centered on zero energy loss. It is typically about 0.2 eV to 2 eV wide and represents primarily the energy spread in the original beam and small energy losses that occur in predominantly elastic collisions between the beam electrons and atomic nuclei. A broad plasmon peak 104 is caused by a resonance of the beam electrons with the valence electrons. FIG. 1B shows peaks 110, 112, and 114 having much higher energy losses that than those shown in FIG. 1A. Each peak is associated with the removal of a specific inner shell electron and the peak is characteristic of the specific sample material. The core loss spectrum provides information that readily identifies materials present in the sample, although information about the sample is also available from low-loss regions of the energy loss spectrum.
FIG. 2A shows a scanning transmission electron microscopic 200 that can simultaneously detect dark-field electrons 202 and perform EELS on bright-field electrons 204. Microscope 200 includes an electron source 210 and a focusing column 212 that focuses electrons from source 210 into a small spot and scans the spot across a thin sample 214. The beam is composed of high energy electrons, that is, electrons having typical energies of between about 50 keV and 1,000 keV. Electrons that pass through sample 214 enter projection optics 216. Projection optics 216 can be set to form a magnified image of the sample 214 at the entrance plane of a spectrometer 217, or to form a diffraction pattern at the entrance plane of the spectrometer 217. For STEM applications, projection optics 216 are typically adjusted to form a diffraction pattern at the entrance plane of the spectrometer so that bright-field electrons 204, which passed through the sample with minimal deflection, pass through an entrance aperture 215 and enter spectrometer 217, while dark-field electrons 202, which were more strongly deflected by the sample, are detected by an annular dark-field STEM detector 218. A signal from annular STEM detector 218 is amplified by an amplifier 220. The annular STEM detector 218 is typically a scintillator-photomultiplier detector or a solid state PIN detector. Bright-field electrons 204 pass through the center hole of annular STEM detector 218 and into spectrometer 217, which includes a prism 222 that disperses the electrons according to their energies into different trajectories 224a, 224b . . . 224e, etc.
Electrons are spread vertically according to their energies in an energy dispersive plane 225. A microscope that is capable of operating in the energy selected imaging mode described above includes an energy-selecting slit 226, having an upper knife edge 226U and a lower knife edge 226L, positioned at or near energy dispersive plane 225. The space between the knife edges is adjustable to pass electrons having energies within different ranges. Electrons 230 that pass through energy-selecting slit 226 are focused by imaging optics 232 onto a detector 234, such as a film, a fluorescence screen, a CCD detector, or an active pixel sensor. Electrons 236 having energies outside the specified range are blocked by energy-selecting slit 226.
Annular detector 218 does not interfere with the bright-field electrons 204 entering the prism 222 because of the annular shape of STEM detector 218 blocks only electrons away from the beam axis. Such a system was not considered suitable for STEM detection of bright-field electrons while simultaneously performing EELS because the bright-field detector and its supports would block the electron beam from entering the prism.
FIG. 2B shows another scanning transmission electron microscopic 248 that can simultaneously detect dark-field electrons 202 and perform EELS on bright-field electrons 204. Microscope 248 includes a spectrometer 250 configured as an “in-column” spectrometer, as opposed to spectrometer 217 of FIG. 2A, which is configured as a “post column” spectrometer. In an “in-column” spectrometer, electrons leave the spectrometer parallel to the direction at which the electrons entered. Spectrometer 250 includes for a prism an “omega filter” that typically includes at least four elements 252A, 252B, 252C and 252D. Elements 252A and 252B offset the electron path and disperse the electron beam. Elements 252C and 252D further disperse the electron beam and displace the beam back to the original optical axis. The symmetry between the first half of the omega filter consisting of elements 252A and 252B and the second half of the omega filter consisting of elements 252C and 252D are configured to cause several aberrations of the prisms to cancel. The dispersive actions of these two halves of the omega filter do not cancel and create an energy dispersive plane 254 after element 252D. In this plane, energy-selecting slits 256L and 256R are positioned. Electrons 260 that exit element 252D are focused by imaging optics 232 onto a detector 234.
It can be noted that both the “low-loss region” and the “core-loss region” contain both bright-field and dark-field electrons, so the electrons that exit the sample can be divided in four categories: bright-field low-loss electrons, dark-field low-loss electrons, bright-field core-loss electrons, and dark-field core-loss electrons. The “zero-loss peak” refers to energy loss and not to deflection angle, and so “zero-loss electrons” are not the same as “bright-field” electrons. For example, electrons that are elastically scattered by atomic nuclei lose very little energy but may be scattered at very large angles. Electrons from all four categories can be used to provide information about the sample. The typical fractions of electrons in the above categories are ˜95%, ˜5%, ˜1%, and ˜0.05%, respectively. Thus, in a typical microscope operation, the bright-field low-loss electrons are the largest fraction of the exiting beam, and the dark-field core-loss electrons are the smallest fraction of the exiting beam. Because the dark-field EELS signal is typically 10 to 100 times smaller than the bright-field EELS signal, the dark-field contribution to the EELS signal is usually neglected. In a typical microscope operation, the projection system between the sample and the annular STEM detector is set so that the dark-field electrons strike the annular STEM detector, and the bright-field core-loss electrons are recorded by the spectrometer. Also, the entrance aperture of the spectrometer normally only passes the bright-field electrons, so the spectrometer records a bright-field EELS signal. The term “EELS” is used herein in its ordinary meaning to refer primarily to bright-field EELS.